1. Field of the Invention
This invention relates to an ink jet recording head which ejects ink droplets to form images on a recording material, and, more particularly, it is concerned with an "ink-on-demand" type ink jet recording head of an improved construction.
2. Description of the Prior Art
FIG. 1 of the accompanying drawing schematically illustrates a conventional "ink-on-demand" type ink jet recording head. In this ink jet recording head, ink is supplied from an ink tank (not shown in the drawing) through a feeding tube 1 into a common liquid chamber 2, from which the ink is further led into an ink discharge flow path 3 of a narrow width. At one portion of the inner bottom surface of this ink discharge flow path 3, there is disposed an electro-thermal energy transducing element 4, at which the ink receives heat as the energy for the ink ejection, and is expelled from an opening 5 at the distal end of the flow path in the form of a droplet 6. (It is to be noted that the drawing does not show signal lines and other component elements for the electro-thermal energy transducing element.) In this "ink-on-demand" type ink jet recording head, feeding of the ink into the discharge flow path 3 by application of a pressure to it is difficult as a practical matter because, among other reasons, that the discharge energy is small. Therefore, ink feeding into the discharge flow path 3 has been effected mainly by the capillary action of the discharge flow path 3 and the surface tension at the opening 5.
According to this manner of ink supply, however, since the force available for supplying ink into the discharge flow path 3 is weak, its feeding quantity decreases due, for example, to a temperature reduction, etc., which makes it unable to keep up with the outlet quantity of the ink from the opening 5 of the flow path 3, thereby causing irregularity in the size of the ink droplets or inability to eject ink. The causes for such troubles will be described in detail in the following.
It has generally been known that, if the viscosity and surface tension of ink are constant, the speed and quantity of ink feeding into the ink discharge flow path 3 assume certain values determined by a constant K of the shape of the ink droplet, a width W, a length L, and a height H of the ink flow path 3.
These values constitute important factors for determining the ink ejection frequency from the ink jet recording head, hence maintenance of constant and appropriate values of these factors is one of the essential requirements for ejecting the ink droplets in an adequate size and a same shape. Usually, however, the ink to be used for this kind of recording head has viscosity-versus-temperature curve as shown in FIG. 2 of the accompanying drawing. In this graphical representation, the ink abruptly increases its viscosity at a temperature lower than about 15.degree. C. to assume a viscosity value which is far removed from a desired viscosity level, and, at a temperature of 5.degree. C. or in its vicinity, the viscosity becomes several times as high as that of the ordinary level. This means that the ink jet recording head having the flow path of constant width W, length L, and height H significantly changes its ejection frequency to a large extent, which causes irregular dot diameter, inability of ink ejection, and other troubles.
On account of this, various measures were taken in the conventional ink jet recording head such that a back-up system is provided in the ink jet recording head, by which a temperature of the whole system is controlled to maintain the ink in its desired viscosity, or a display device is provided to indicate the inability of the ink jet recording head to operate. With such measures being taken, however, as a matter of practice, the inability of use of the ink jet recording head at a low temperature or a high temperature condition gives rise to serious problems, and the installation of the back-up system also brings about various difficult problems such as increase in the manufacturing cost of the apparatus, maintenance of the apparatus, and so forth.